Nacreous or pearlescent pigments which are titanium dioxide coatings on mica substrates are well known. The pigments exhibit pearl-like and/or iridescent effects from their reflection and transmission of light. The titanium dioxide coating is actually transparent to light. However, because the coatings are extremely smooth and have a high index of refraction, they follow the laws of thin films. Part of the light which strikes each platelet is reflected and part transmitted to lower platelets where multiple reflections can occur. These multiple reflections from lower layers give a sense of depth or sheen which simulates the real pearl. Also, if the thickness of the titanium dioxide layer is controlled, interference of light occurs and the platelets act as optical filters separating light into two components. A color is seen by reflection and a complementary color by transmission.
Pearlescent pigments are used extensively in a variety of applications including plastic incorporation, automotive coatings and in cosmetics. The pearlescent pigments which are titanium dioxide coated on a mica substrate have a high index of refraction. The pigments are normally dispersed in mediums such as paint films, or nail enamel films which, when fully cured, have an index of refraction of about 1.5. The index of refraction of the pearlescent pigment must therefore be considerably higher than 1.5 if reflectivity of light is to occur. This high index of refraction is provided by the titanium dioxide layer whose index can vary between 2.3 and 2.5. The mica substrate on which the titanium dioxide is coated has an index about 1.5 and therefore, does not take part in any reflectivity when incorporated in a film. The rutile form of titanium dioxide has a higher index than the anatase form and as a result, the rutile modification will have greater reflectivity than the anatase form. Therefore, the rutile modification of titanium dioxide in a pearlescent pigment is more desirable than the anatase modification.
There are many other reasons for preferring the rutile modification. The rutile modification is more stable in outdoor weathering than is the anatase modification. The rutile modification of a titanium dioxide coated mica results in a product which has better luster and reflectivity, better color and color homogeneity and also contains fewer small particles. In the processing stage during the formation of the titanium dioxide on the mica, particles which are not attached to the mica may form. These small particles, which resemble pigmentary TiO.sub.2, cause light scattering. If too many small particles are present, the pearlescent appearance may be lost or diminished. The process for coating mica in the rutile crystalline form results in very few small particles compared to the anatase form.
If mica is coated with a layer of hydrous titanium dioxide and then subjected to the normal processing methods which include washing drying and calcining usually from 750.degree. C. to 900.degree. C. the titanium dioxide which is formed is in the anatase form. The presence of the mica causes the TiO.sub.2 to orient in the anatase crystalline form. Such pigments have been described for example in Quinn et al. U.S. Pat. No. 3,437,515 issued Apr. 8, 1969 and Rieger et al U.S. Pat. No. 3,418,146 issued Dec. 24, 1968 and Linton U.S. Pat. No. 3,087,828.
If a rutile crystalline form is desired, a layer of hydrous tin oxide is first precipitated on the surface of the mica followed by a layer of hydrous titanium dioxide. When this layered combination is processed and calcined, the titanium dioxide is oriented in the rutile form. This is described in detail in U.S. Pat. Nos. 4,038,099 and also 4,086,100. Other methods of forming rutile TiO.sub.2 on mica substrates using tin oxide are also described.
Although many additives can aid in the formation of rutile TiO.sub.2 per se, the formation of rutile TiO.sub.2 on mica requires a very special additive. The coating of TiO.sub.2 on the mica must be smooth and uniform. If an irregular surface is formed, light scattering takes place and the pigment no longer functions as a pearlescent pigment. The coating of TiO.sub.2 must also adhere strongly to the mica or else the coating of TiO.sub.2 will be separated from the mica during processing, resulting in considerable breakage and loss of luster. It is also necessary that the luster, color, and color homogeneity be maintained. Small particle formation must be suppressed. Otherwise, the small particles will scatter light and diminish the pearlescent luster as was mentioned previously. An additive which is used must therefore perform many functions besides being a rutile crystalline director. It has been difficult to find an additive (other than tin) which can orient the TiO.sub.2 to the rutile modification while still maintaining quality and all of the other desirable characteristics.
Presently, additives other than tin for forming the rutile modification of titanium dioxide on mica while still maintaining all other desirable characteristics do not exist. The use of tin oxide is the method of choice and is used universally in all commercial rutile titanium dioxide coated micas.
There are, however, two major disadvantages to the use of tin to make rutile TiO.sub.2 coated mica. The first is that tin oxide is not permitted in polymer compositions which are to be used in contact with food. Thus, any high quality pearlescent or interference pigment which contains tin oxide cannot be used to color the polymer film. The second is that in some countries, notably Japan, the presence of tin oxide is not permitted in cosmetic products. Cosmetic manufacturers are therefore faced with a choice of either formulating cosmetic products destined for Japan with anatase only products and having a second line of the same products for the rest of the world formulated with rutile products or having a single anatase product line for the entire world. The result is that polymer formulations in contact with food and cosmetic lines to be used worldwide use anatase products even though the rutile TiO.sub.2 -coated products have better color, color homogeneity and luster.
It is therefore the object of this invention to provide a pearlescent pigment of titanium dioxide coated mica in which the titanium dioxide is in the rutile crystalline form and in which tin has not been used to promote rutilization. A further object of this invention is to provide a rutile TiO.sub.2 -coated mica which has the same advantages and characteristics of the tin containing product which includes luster, color, color homogeneity and few small particle formation during manufacture. These and other objects of the invention will become apparent to one of ordinary skill in this art from the following detailed description.